How to apply textures with alpha channel?
It looks like the ground that you are trying to apply the texture to is on the Background layer and is being used as a sort of a mask, so it is not being rendered.  What I did to fix your problem is duplicate the ground, move the duplicate in the Z direction just slightly to avoid it intersecting with your ground that is being used as a mask, and then move the duplicated ground onto the foreground layer, which you have defined as the first layer, by pressing M.


Now the ground will render.  Next, you need to apply the cracks as a texture to the new ground.  Applying textures with alpha channels is simple using cycles.  Here is the node setup that is used:

 

Now if you render your should be able to see the cracks.   

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